The invention relates to a helical scan tape recorder such as a video tape recorder, having at least one magnetic head mounted on a head support, rotatable about a drive spindle axis; and in particular to such a recorder whose head support is detachable from the drive spindle. The support includes an elastically deformable sleeve which is deformed to clamp against the drive spindle by a clamping ring.
Such an apparatus is known from Austrian patent specification AT-PS 345577. In this known apparatus the head disc consists of a cylindrical head drum which constitutes the rotary part of a drum unit for guiding the magnetic tape in a helical path. In addition to the rotary head drum the drum unit further comprises a stationary drum which is coaxial therewith and which is spaced therefrom by a narrow gap only. The stationary drum comprises a guide edge for the magnetic tape, which extends helically over a part of the circumference. Owing to its rotation the rotary head drum locally produces a film of air between the magnetic tape and the drum unit. As a result of this the friction between the magnetic tape and the drum unit is reduced. On its side which is remote from the stationary drum, the head drum includes an integral clamping sleeve having a cylindrical outer surface. A clamping ring, formed with a slot in the ring wall, is arranged around the clamping sleeve so that the parts of the clamping ring which are spaced from each other by the slot can resiliently be moved towards each other. On the one side of the slot a screw bolt is fitted in the clamping ring and extends with clearance through the clamping ring on the other side of the slot. By tightening the screw bolt the clamping ring is deformed between its released position and its clamping position, so that the clamping sleeve of the head drum is also deformed and the head drum is centered and clamped on the drive spindle.
For a most economic use of the magnetic tape in video recorders the magnetic tape is provided with a pattern of adjacent signal tracks which is as dense as possible. With modern video recorders slightly overlapping signal tracks are used having a width of approximately 40 microns and having a distance between the centers of the tracks of approximately 30 microns. The track length is approximately 100 mm. Over the entire length the tracks should be situated within very narrow tolerances, in particular because great value is attached to the possibility of being able to play back on one video recorder programs recorded on other video recorders without a noticeable loss of quality. A really successful interchangeability of cassettes, provided with recorded programs, and video recorders is obtained only if a video programs recorded on one video recorder can be played back on another video recorder without a perceptible loss of quality of the signal reproduced by the video recorder. This requirement is referred to as the compatibility of video recorders and magnetic tape cassettes, together belonging to a standard system for recording and reproducing video signals. As a result of this only very small tolerance ranges are available for the position of the magnetic heads relative to the stationary tape guide edge. In video recorders, which are equipped with a plurality of magnetic heads, such as helical scan video cassette recorders adapted for domestic use, it is essential that the two heads are also accurately positioned relative to each other in view of the synchronism of the signals which are recorded or read by the two heads. Also in this case only small tolerance ranges are available, because otherwise visible time errors will appear in the reproduced video picture, i.e. instabilities will occur near the edge of the picture.
In view of the stringent accuracy requirements imposed on the position of the magnetic heads, special optical precision instruments are generally used for mounting the magnetic heads on the head support. In the case of detachable head supports for helical scan recorders described above centered the head support is first centred and mounted on a special mounting spindle with the aid of its own clamping device. After mounting and adjustment of the magnetic heads the head support is removed from the mounting spindle by releasing the clamping device, after which the head support may be mounted on the drive spindle of a video recorder. Obviously, it is essential that the adjustments performed with the aid of the special mounting spindle are most accurately reproduced after the magnetic head support has been mounted on the drive spindle of a video recorder. In this respect a disadvantage of the known apparatus is that owing to the nature of the clamping device used the head support is clamped on two parts of the periphery of the clamping sleeve which are not predetermined. The location of these parts is not predetermined, which will give rise to an eccentricity of unpredictable magnitude and orientation when the head support is mounted on a drive spindle. These effects are caused by the fact that when the split clamping ring is tightened the clamping sleeve is deformed more or less to an oval, until the clearance between the clamping sleeve and the drive spindle is eliminated locally. In the bore of the head support, which has been deformed to an oval, the drive spindle is disposed at a location which depends on random factors, such as the initial position of the drive spindle in the bore. Another disadvantage of the known video recorder is that both mounting and removal of the head support in and from a video recorder is complicated by the fact that the screw bolt of the clamping ring is manipulated in a direction perpendicular to the axis of rotation of the drive spindle.